screenshots, and maybe translation tests, run only under `en_US`

[cfm]

Description

At least some of the translation test suite runs with only en_US available, so that's the language it uses in practice.

Steps to Reproduce

$ securedrop/bin/dev-shell bin/translation-test de_DE

Expected Behavior

Tests run and screenshots are generated in de_DE.

Actual Behavior

Tests run and screenshots (e.g.) are generated in en_US, with no other languages available for selection

Comments

Flagged by @legoktm in https://github.com/freedomofpress/securedrop/pull/7240#pullrequestreview-2352491308.

[cfm]

Fixtures were parameterized incorrectly at both ends of this end-to-end testing:

• In 4db4bd72c5cefdc480ae0a22287840f765f2776d, the server fixture was missing SUPPORTED_LOCALES, so it provided only the default en_US.
• In 7edff795dd497a527dcea8e282b7c5ce3b664b81, the client fixture was set up before the test was parameterized.

Both fixes are proposed as part of #7240.